I. Field of the Invention
The present invention relates to the manufacture of devices for the detection of high-energy electromagnetic radiation (X- and y-rays). More particularly, the present invention relates to the manufacture and the use of the high-spectral resolution virtual Frisch-grid radiation detectors based on CdZnTe, CdTe, CdMnTe, HgI2, TlBr, or other semiconductors capable of operating as single-carrier-transport devices.
II. Background of the Related Art
Semiconductor nuclear radiation detectors have experienced a rapid development in the last few years. They are now used in a large variety of fields, including nuclear physics, X-ray and gamma ray astronomy, and nuclear medicine. Their imaging capabilities, good energy resolution, and the ability to fabricate compact systems are very attractive features, in comparison with other types of detectors, such as gas detectors and scintillators. In recent years, a substantial effort has been invested in developing a range of compound semiconductors with wide band gap and high atomic number for X-ray and gamma ray detectors. These compound semiconductors are generally derived from elements of groups III and V (e.g. GaAs) and groups II and VI (e.g. CdTe) of the periodic table. However, besides binary compounds, ternary materials have been also produced, e.g. CdZnTe and CdMnTe. Among these compound semiconductors, cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) are two of the most promising materials for radiation detectors with good energy resolution, high detection efficiency, and room temperature operation.
The drawback of the compound semiconductor based detectors of the prior art is that the amplitude of output signal is affected by the immobile holes left at the interaction point produced by the incident gamma ray. Since gamma rays interact randomly inside the detector's volume, the output signals depend on the locations of interaction points. Such behavior of the output signals, caused by immobile holes, is called the induction effect. This effect degrades spectral resolution of semiconductor detectors unless special measures are implemented to neutralize the holes.
There are two common ways to minimize the induction effect: (1) subtracting the fraction of the charge signal contributed by the stationary holes and (2) electrostatic shielding of the stationary holes. The validity of both techniques is consistent with the Ramo-Shockley theorem (S. Ramo, Proc. IRE 27, p. 584, 1939; W. Shockley, J. Appl. Phys. 9, p. 635, 1938; each of which is hereby incorporated herein by reference in its entirety).
In the first technique, the induced charge contributed by the holes is measured and electronically subtracted from the total output signal. (USSR Patent No. SU-1264723A; incorporated herein by reference in its entirety). The holes-induced signal can be measured with one or several electrodes adjacent to the collecting one. The electrodes can be comprised of pixels or strips and, depending on the locations of interaction points, the same electrode can be used for measuring collected or induced-only charges. The coplanar-grid devices proposed for CdZnTe, (P. N. Luke, Appl. Phys. Lett. 65 (22), pp. 2884-2886, 1994; U.S. Pat. No. 5,530,249; each of which is hereby incorporated herein by reference in its entirety), and fluid Xe detectors (A. Bolotnikov, et al., IEEE Trans. Nucl. Sci., Vol. 51, n. 3, pp. 1006-1010, 2004; incorporated herein by reference in its entirety), are special cases of using this technique.
The second technique is based on developing an electrostatic shielding of the stationary holes. There are two types of devices which employ the electrostatic shielding: Frisch-grid and virtual Frisch-grid detectors. In the first type detectors, e.g., in a classic Frisch-grid ionization chamber, a metal grid (or mesh) is used for electrostatic shielding of the collecting electrode (or several collecting electrodes) from the positive ions (holes in the case of semiconductors). In the second type detectors, i.e., the virtual Frisch-grid detectors, the special grounded (or virtually grounded) electrode (or several electrodes) is made to produce essentially the same shielding effect as if a real Frisch-grid were placed inside the detector. In the past, several designs of the virtual Frisch-grid detectors were proposed for CdZnTe semiconductor: pixel detectors (H. H. Barrett, et al., Phys. Rev. Lett. 75 (1), p. 156, 1995; incorporated herein by reference in its entirety), CAPture™ (K. Parnham, et al., in Hard X-Ray, Gamma-Ray and Neutron Detector Physics, Proceedings of SPIE, 1999; incorporated herein by reference in its entirety), hemispherical, (C. Szeles, et al., in Hard X-Ray and Gamma-Ray Detector Physics VIII, edited by Larry A. Franks, et al., Proceedings of SPIE Vol. 6319 (SPIE, Bellingham, Wash., 2006); incorporated herein by reference in its entirety), and Frisch-ring, (U.S. Pat. No. 6,175,120; G. Montemont, et al., IEEE Trans. Nucl. Sci., Vol. 48, pp. 278-281, 2001; each of which is incorporated herein by reference in its entirety).
Pixel Detectors
In the pixel detectors, the charge signal induced on a given pixel by the stationary charges is greatly reduced, because it is shared between other pixels. Therefore, for each individual pixel the other pixels (virtually grounded) act as an electrostatic shield (it is also called the “small-pixel effect”).
CAPture™ and hemispherical detectors
The CAPture™ and hemispherical detectors as shown in FIG. 1A are produced by extending the cathode electrode up the sides of the detector body 100. In both devices, the extended electrodes are in a physical contact with the semiconductor surfaces. As a result, a wide area of the bare surface surrounding the anode is required to keep the surface leakage current below an acceptable level.
Frisch-Ring Detectors
As shown in FIG. 1B, in the existing Frisch-ring detectors (U.S. Pat. No. 6,175,120, Montemont, 2001; Bolotnikov, 2006), the cathode is also extended up the sides of the detector as in the CAPture™ and hemispherical detectors (see FIG. 1A), but the extended portion of the cathode is physically separated from the semiconductor surfaces by a thin layer of insulating material. The non-contacting electrode (also known as the non-contacting Frisch-ring; U.S. Pat. No. 6,175,120; incorporated herein by reference) is the main feature that makes Frisch-ring detectors different from CAPture™ and hemispherical-type devices.
In the Frisch-ring detectors the non-contacting electrode is placed on the side surfaces of the crystal and physically connected to the cathode, as illustrated in FIG. 1B. (U.S. Pat. No. 6,175,120, Montemont, 2001; Bolotnikov, 2006). A common high-voltage bias (with respect to the anode) is applied to the cathode and the ring, while the output signal is read out from the anode. This configuration requires a gap of about 2-3-mm of the unshielded surface to be left near the anode's contact to prevent high leakage current or even possible discharge in the area between these two electrodes. As a result, an electrostatically unshielded area of the surface exists near the anode.
Overall, previously known designs of virtual Frisch-grid detectors (e.g., FIG. 1A and FIG. 1B) have two common problems that affect their spectroscopic performances. The first problem is the presence of the unshielded area near the anode (see FIG. 1A and FIG. 1B) and the second problem is related to the fact that the original virtual Frisch-grid detectors are essentially two-terminal devices in which the cathode signal cannot provide the particle's interaction depth information to correct the electron losses due to electron trapping (Bolotnikov, 2006). For thin detectors the electron trapping has little effect on the detection performance and can be neglected. However, this problem must be solved for thick detectors longer than about 10 mm, usually employed by virtual Frisch-grid detectors, where electron trapping can be significant.
Recently, a new design of virtual Frisch-grid detectors has been proposed to address the above problems, which lead to improved performance of this type of devices (Bolotnikov, et al., in Proceedings of SPIE Hard XRay and Gamma-Ray Detector Physics VIII, Vol. 6702, edited by L. A. Franks, et al., (SPIE, Bellingham, Wash., 2007); incorporated herein by reference). A schematic of the device is shown in FIG. 1C. A rectangular shaped crystal (bar) 102 has the geometrical aspect ratio (a ratio of its length to its width) of 2 or more as in the existing Frisch-ring devices, e.g., FIG. 1B (Bolotnikov, 2006). The cathode metallization 101 is extended 2-3 mm up to the side surfaces. The non-contacting ring 104 covers the device's side surfaces up to the cathode's 101 edge and is kept at the same potential as the anode 105. The insulating layer 103, e.g., the ultra-thin polyester shrink tube, covers the entire area of the side surfaces. This provides decoupling of the non-contacting ring 104 and the cathode 101 and, at the same time, allows one to apply high differential bias (up to 3000 V and higher) between the cathode 101 and the shielding electrode 104 (non-contacting ring). In turn, the decoupling of the cathode allows for implementing the cathode readout scheme, and enables those skilled in the art, first, to correct for electron trapping and, second, to reject the events interacting near the anode 105 which contribute to the background.
However, this design is not optimal for achieving the full capabilities of the cathode readout scheme to correct the charge loss due to trapping. There are contradicting requirements in this approach. To ensure strong shielding effect of the virtual Frisch grid, the shielding electrode on the crystal's side should cover the entire area of the device's surface as shown in FIG. 1C. But when the entire detector is shielded, very few signals can be detected by the cathode. This makes the cathode insensitive to the events interacting deep inside the detector. On the other hand, it is critical for the good detector performance to be able to detect all events within the crystal and even those that interact close to the anode.
Therefore, it will be desirable to have a virtual Frisch-grid detector(s) and an array of virtual Frisch-grid detectors that effectively reduce the electron trapping problem in the moderately to substantially thick detector crystals, while avoiding the shortcomings of the prior art.